Heat oven to 425 degrees F and line a broiler-proof rimmed baking sheet with nonstick foil. In a bowl, combine the mustards. In a second bowl, combine the panko, half the tarragon (about 1 tablespoon), and 1 tablespoon oil.

Heat the remaining 2 tablespoons oil in a large skillet over medium heat. Add the potatoes to the pan, cover and cook, tossing occasionally, until golden brown and tender, 12 to 14 minutes.

Meanwhile, place the fish on the prepared baking sheet. Spread the mustard mixture on top of the fish then sprinkle with the bread crumb mixture, pressing gently to adhere. Roast until the fish is opaque throughout, 10 to 12 minutes. Increase heat to broil and broil until golden brown, 1 minute.

Uncover the skillet, drizzle the vinegar over the potatoes and toss to coat evenly. Add the peas and cook until heated through, about 2 minutes. Remove from the heat and toss with the remaining tarragon. Serve with the fish.

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